Chlorofluorocarbons (CFC's) are widely used solvents for precision cleaning of parts and components due to their superior physical and chemical properties, especially their solvency for contaminating materials such as oils, greases, resin fluxes, particulates, and other contaminates. One solvent commonly used in many applications is CFC-113 (1,1,2-trichloro-1,2,2-trifluoroethane). These solvents are used to clean and/or degrease components or systems related to, but not limited to, oxygen handling systems, refrigeration equipments or heat pumps, electronics, implantable prosthetic devices, and optical equipment. In addition, these solvents have been used as a means to measure residue remaining is a system. For example, in Air Force launch vehicle applications involving liquid or gaseous oxygen systems, CFC-113 was the solvent of choice used to detect and quantify the amount of hydrocarbon and fluorocarbon residues in these systems, since the presence of those contaminants can be catastrophic. A further application of these solvents is for foam blowing and polymer coating.
CFC-113 has many favorable characteristics such as low toxicity; non-flammability; and stability. Furthermore, CFC-113 is not classified as an air-polluting volatile organic compounds (VOC's) by environmental regulators, is practically odorless, and has a high worker exposure threshold value, eliminating the need for costly air circulation or dilution precautions. These benefits also came at a low price (less than 1% of total manufacturing costs in 1988). Coupled with the growth of the electronics industry, and concerns over worker safety due to toxic chemical exposure and hazardous waste disposal resulting from the use of VOC's, the desirable characteristics led to the widespread use of CFC-113.
With the rise of electronic equipment during the 1970s, the need to properly clean these contaminant sensitive parts became very important and the solvent, 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113), was found to be an excellent and versatile solvent. Being able to dissolve an unusually large array of contaminants (greases, oils, etc) and having excellent physical characteristics, CFC-113 became the ‘solvent-of-choice’ for electronics cleaning and it's use spread to other applications—especially military. Specifically, CFC-113 was used to remove solder flux from small spaces between electronic components so as to ensure adhesion of coatings, and prevent corrosion and electromigration of ions. Even more favorable were the non-aggressive properties of CFC-113 towards most polymers and coatings and its use permitted a wide use of plastics and other solvent-sensitive materials in the manufacture of electronic components. By 1986, the removal of solder flux from printed circuit board assemblies accounted for close to half of worldwide CFC-113 consumption. A significant portion of the remaining half was utilized by the military and in particular, aviation.
The use of CFC-113, however, is restricted due to the Montreal Protocol due to its ability to react and deplete atmospheric ozone. By the Mid 1980s, problems regarding the ozone became apparent and the primary culprits were certain halogenated hydrocarbons including CFC-113. In 1987, twenty-four nations agreed in principle to control ozone-depleting substances (ODS), such as CFC-113. Although this solvent had become critical to the electronics industry, the importance of protecting the earth's ozone layer weighed heavier. Thus, non-toxic and non-ozone depleting replacement solvents became a priority for electronics manufacturers and the military. Various CFC-113 substitutes have emerged and often rely on solvents such as n-propyl bromide and dichloroethylene, which are flammable and not as desirable as CFC-113.
Refrigeration systems also require periodic flushing to remove contaminants. A contaminated refrigeration system may have drastically reduced performance resulting from compressor failure, for example. The materials and contaminants in these systems differ from other applications and therefore solvents must be optimized accordingly. For example, a flushing solvent must be compatible with the elastomers and metals in typical systems, while at the same time have the solvency properties to remove oils, acids, and decomposition products of the oils and refrigerants. Some of the currently used flushing solvents include terpenes (e.g., d-limonene), n-propyl bromide, pentafluorobutane, HCFC-141b, and HCFC-225 ca/cb.
Selection for CFC replacements typically involves two steps. First, commercially available materials with limited impact on the environment are selected; these are termed next-generation replacements. These next-generation replacements are interim and do not have all the desired properties of an ideal replacement (e.g. they are not as effective solvents or have non-zero ozone depletion potentials, or ODP). The second step is to evaluate the so-called second-generation replacements that are not commercially available, but are only available in research quantities or by custom synthesis, and have properties that are not known. Evaluation and manipulation (e.g. by mixing) of these candidate second generation solvents will result in second generation replacements that meet or exceed the next generation solvents' overall performance since all critical properties required of the solvent are accounted for.
Many factors are important when selecting CFC second-generation replacement solvents. Some of the critical performance properties for a second-generation CFC replacements include: cleaning effectiveness or solvency, volatility (e.g., Boiling point), compatibility with materials to be cleaned (e.g. metals, elastomers and systems), toxicity (e.g., LC50, LD50, cardiac sensitization, mutagenicity, skin irritation), environmental persistence (e.g., ozone depletion potential (ODP), global warming potential (GWP), tropospheric lifetime (TLT), biodegradability), flammability (e.g., autogenous ignition temperature (AIT), flash point), cost and availability.
The solvency of the replacement should be comparable to CFC so the primary factor of performance is not compromised. The volatility and materials compatibility of the replacement solvent should be similar to the CFC so there is minimal impact on existing cleaning systems by switching solvents. Hazardous risks such as flammability, toxicity, and environmental impact are also critical since every manufacturer will be required to eliminate hazardous solvents in the near future.
The solvency performance of the candidate replacements can be quantified through the solubility parameter of the compounds. The hazard potential of the candidate replacements can be characterized using toxicity information such as lethal doses (LD), lethal concentrations (LC) or threshold limit values (TLV), and flammability information. Environmental properties can be analyzed through ozone depletion potential (ODP), global warming potential (GWP), and tropospheric lifetime (TLT). For a discussion of these parameters and their measurements or calculations, see e.g. U.S. Pat. No. 6,300,378, to Tapscott. Volatility can be assessed using the normal boiling point (nBP) of the solvent. If all of these properties and others can be experimentally measured or modeled, one could identify and test non-hazardous “drop-in” replacement solvents to replace hazardous solvents. The following paragraphs discuss the relevance of these performance parameters.
Cleaning Effectiveness or Solvency
The solubility parameter is a very important measure of the cleaning effectiveness of a solvent in dissolving and removing another material. In general, these parameters provide an easy numerical method of rapidly predicting the extent of interaction between materials, particularly liquids. Compounds with similar solubility parameters are known by those skilled in the art to have similar solvency properties. For example, CFC-113 has a solubility parameter or about 7.5 which is within the range where a solvent will dissolve both hydrocarbon and fluorocarbon greases. This is a fairly unique solubility parameter and is a major part of what makes CFC-113 such an excellent solvent. It also makes the substitution for CFC-113 rather difficult.
A quantitative method for comparing the relative solubility of different materials is through the use of solubility parameters. This concept of expressing solubility is based on the idea that the solution of one material in another is a spontaneous process, and that it can be stated in terms of the free energy of mixing as shown below:ΔG=ΔH+TΔS,  (1)where ΔG is the free energy of mixing, ΔH is the enthalpy of mixing, and ΔS is the entropy of mixing. The controlling term for a spontaneous process (where ΔG is negative) is the enthalpy of mixing, which can be expressed in terms of x1 and x2, the mole fraction of the components, V1 and V2, the molar volumes, and a1 and a2, the interaction constants.
The expressions for the enthalpy and entropy of mixing are given below:
                              Δ          ⁢                                          ⁢                      H            m                          =                                                                              x                  1                                ⁢                                  x                  2                                ⁢                                  V                  1                                ⁢                                  V                  2                                                                                                  x                    1                                    ⁢                                      V                    1                                                  +                                                      x                    2                                    ⁢                                      V                    2                                                                        ⁡                          [                                                                                          a                      1                                                                            V                    1                                                  -                                                                            a                      2                                                                            V                    2                                                              ]                                2                                    (        2        )                                          Δ          ⁢                                          ⁢                      S            m                          =                  R          ⁡                      [                                                            x                  1                                ⁢                ln                ⁢                                                                  ⁢                                  x                  1                                            +                                                x                  2                                ⁢                ln                ⁢                                                                  ⁢                                  x                  2                                                      ]                                              (        3        )            
The cohesive energy of a mole of a liquid mixture can be stated as
                                          Δ            ⁢                                                  ⁢                          E              m                                =                                                                      (                                                                                    x                        1                                            ⁢                                              V                        1                                                              +                                                                  x                        2                                            ⁢                                              V                        2                                                                              )                                ⁡                                  [                                                                                    (                                                                              Δ                            ⁢                                                                                                                  ⁢                                                          E                              1                              v                                                                                                            V                            1                                                                          )                                                                    1                        /                        2                                                              -                                                                  (                                                                              Δ                            ⁢                                                                                                                  ⁢                                                          E                              2                              v                                                                                                            V                            2                                                                          )                                                                    1                        /                        2                                                                              ]                                            2                        ⁢                          ϕ              1                        ⁢                          ϕ              2                                      ,                            (        4        )            where ΔEν is the energy of vaporization and φ1 and φ2 are volume fractions. The enthalpy of mixing can be rewritten as
                                          Δ            ⁢                                                  ⁢                          H              m                                =                                                                      V                  T                                ⁡                                  [                                                                                    (                                                                              Δ                            ⁢                                                                                                                  ⁢                                                          E                              1                              v                                                                                                            V                            1                                                                          )                                                                    1                        /                        2                                                              -                                                                  (                                                                              Δ                            ⁢                                                                                                                  ⁢                                                          E                              2                              v                                                                                                            V                            2                                                                          )                                                                    1                        /                        2                                                                              ]                                            2                        ⁢                          ϕ              1                        ⁢                          ϕ              2                                      ,                            (        5        )            where the term ΔEν/V, the energy of vaporization per unit volume, is a measure of the internal pressure.
This term is called the solubility parameter, δ, and is defined below:
                                                        δ              =                            ⁢                                                (                                                            Δ                      ⁢                                                                                          ⁢                                              E                        v                                                              V                                    )                                                  1                  /                  2                                                                                                        =                            ⁢                                                (                                                                                    Δ                        ⁢                                                                                                  ⁢                                                  H                          v                                                                    -                      RT                                        V                                    )                                                  1                  /                  2                                                                                                                        =                                ⁢                                                      a                                          1                      /                      2                                                        V                                            ,                                                          (        6        )            where ΔHν is the latent heat of vaporization. (The units of the solubility parameter are typically expressed in (cal/cm3)1/2).
Therefore, the free energy of mixing is given by:ΔG=V[δ1−δ2]φ1φ2+RT[x1 ln x1+x2 ln x2]  (7)and solution should occur as δ1 approaches δ2.
The above expression shows that the solubility parameter of a compound can be calculated directly from the latent heat of vaporization and the molar volume of the compound if these are available. Regardless of the method of determination, solubility parameters are useful in comparing the solvency of compounds because solvents with similar solubility parameters are known by those skilled in the art to have similar solvency properties.
For reference, the solubility parameter in (cal/cm3)1/2 for some common compounds are: water, 23.37; acetone, 9.646; ethyl alcohol, 12.779; HFC-134a, 8.067; propane, 6.404; hexane, 7.284; benzene, 9.142; isopropyl alcohol, 11.450; and d-limonene, 8.243.
Volatility
The volatility of a replacement solvent can be described in terms of properties such as the normal boiling point (nBP). An effective solvent replacement must be volatile enough to evaporate, but should not flash off of surfaces since the solvent must reside on the contaminants long enough to dissolve them. An nBP around 40° C. or higher is generally acceptable for cleaning applications.
Compatibility
Material and system compatibility is another requirement for a second-generation solvent. The solvent must be compatible with metals such as aluminum, copper, carbon steel and stainless steel, as well as elastomers. The solvent should not degrade or corrode surfaces in the system being cleaned. The solvent also needs to be compatible with the particular system application. For example, a solvent to be used for cleaning oxygen handling system must be compatible with liquid and gaseous oxygen. In this case, tests such as ASTM G86 for ignition sensitivity to mechanical impact must be considered.
Flammability: Autoignition, Flashpoint
Whether a solvent is suitable as cleaning solvents for systems (e.g., oxygen handling systems) is partially dependent upon its flammability, which sometimes is quantified by the autogenous ignition temperatures (AIT). AIT provides a measure of the material's relative ease of ignition and indicates the approximate temperature at which a material could be expected to spontaneously ignite in high-pressure oxygen. This test is typically performed per ASTM Method G72. A rating system has been established by the NASA White Sands Test Facility and Wright-Patterson Air Force Base. By this system, compounds are classified as A (not recommended, AIT<250° F.), B (caution when used, 250° F.<AIT<400° F.), and C (recommended, AIT>400° F.).
Another aspect of the flammability determination is the flashpoint of the solvent. The flashpoint is the temperature at which a liquid gives off vapor sufficient to form an ignitable mixture with air (oxygen) near the surface of the liquid. The ideal replacement solvent should not have a flashpoint below or at its boiling point. This insures a wide range of conditions whereby the solvent can be safely used.
Environmental Persistence
The environmental persistence of a solvent is also very important. Parameters such as the ozone depletion potential (ODP), global warming potential (GWP), and tropospheric lifetime (TLT) are measures of this attribute. ODP and GWP give the relative ability by weight of a chemical to deplete stratospheric ozone and to contribute to global warming, respectively. Values for ODP, GWP and TLT are calculated based on an earth surface release and then reported relative to a reference compound (typically CFC-11 for ODP and CFC-11 or carbon dioxide for GWP). Generally, the ODP should be less than 0.02, and the GWP and TLT should be minimized, preferably lower than the solvent being replaced.
The biochemical oxygen demand (BOD) is also another measure of persistence typically in groundwater, lakes, and other bodies of water.
Toxicity
Toxicity is yet another factor which must be considered when selecting second-generation replacement solvents. Parameters such as the lethal dose 50 (LD50), lethal concentration 50 (LC50), cardiac sensitization, skin irritation, and mutagenicity (e.g., via the Ames test) can be used as measures. LDn or LCn abbreviation, where n is the percent lethality, is used for the dose of a toxicant lethal to n % of a test population. For instance, at LD50, 50% of the recipients of that particular toxic dose would die. Cardiac sensitization is a measure of the ability of a compound to cause cardiac arrhythmia under stress. Generally, it is desired to minimize these parameters and select compounds that have lower values than the solvent that is being replaced.
Review of Prior Art
The CFC-113 replacements known in prior art do not address all of the required second-generation solvent properties. CFC-113 replacements and solvents that address ozone depletion have been introduced and are disclosed in e.g. U.S. Pat. Nos. 5,035,828, 6,402,857, 6,297,308, and 6,020,298. Various solvents and solvent mixtures are disclosed which have low ODPs. These replacement solvents, however, do not possess all of the desired properties of CFC-113 such as flammability, toxicity, oxygen compatibility and cleaning effectiveness.
In U.S. Pat. No. 5,035,828, HCFC-234 is combined with an aliphatic alcohol or cyclohexane, but this mixture is easily flammable. U.S. Pat. No. 6,402,857 utilizes n-propyl bromide with other organic constituents, which are also flammable and have a significant adverse impact on ozone. U.S. Pat. No. 6,020,298 utilizes hydrofluoropolyethers, and U.S. Pat. No. 6,297,308 utilizes halogenated ethers and hydrocarbons with a surfactant. While these solvents appear to avoid damage to the ozone layer, the perfluorinated compounds contained therein are known to be potent greenhouse gases. In addition, perfluorinated and fluorinated (no chlorine) solvents are undesirable as they can have widely varying solubility properties and different interactions with organic residues when compared to CFC-113.
U.S. Pat. No. 6,103,684 teaches the use of azeotrope-like mixtures comprised of 1-bromopropane with non-halogenated alcohols and alkanes, as well as halogenated alkanes and fluorinated ethers. The ODP for 1-bromopropane is stated as being between 0.002 and 0.03, classifying it as a Class II Ozone Depleting Substance. The flammability limits of 1-bromopropane are 2.7-9.2% in air, with an auto-ignition temperature of 490° C. In addition, the solubility parameter of 1-bromopropane is also 8.839, too high to effectively dissolve many greases and oils. Furthermore, the alcohols and alkanes of this invention are also flammable.
In U.S. Pat. No. 6,048,832, the inventors disclose the use of 1-bromopropane with 4-methoxy-1,1,1,2,2,3,3,4,4-nonafluorobutane (an ether) and at least one other non-halogenated organic compound. As in U.S. Pat. No. 6,103,684, the use of 1-bromopropane is questionable due to its high ODP, flammability, and undesirable solubility parameter. The other components, such as ethanol and 2-propanol, also have high solubility parameters of about 11-13, thereby decreasing the usefulness of these mixtures for a broad spectrum of contaminants as will be taught by the present invention.
Solvents that meet the environmental restrictions and are non-flammable are disclosed in U.S. Pat. Nos. 6,300,378 and 5,759,430 and in Tapscott & Mather, 2000, Tropodegradable fluorocarbon replacements for ozone-depleting and global-warming chemicals. J. Fluorine Chemistry 101:209-213. Compounds disclosed therein are generally non-flammable and/or non-ozone depleting, as they are “tropodegradable fluorocarbons,” defined as compounds having structural weaknesses to ensure rapid decay in the troposphere. When this class of compounds is exposed to sunlight (photolysis) or chemical radicals (e.g. hydroxyls) in the atmosphere, they decay into forms that do not damage the ozone layer nor contribute to the greenhouse effect. The structural weaknesses can take such forms as hydrogen being present on the molecule, a carbon-carbon double bond that is vulnerable to reactions, an ether bond, or a bromine atom being present for easy degradation. These structural vulnerabilities render the molecules unstable, and within a fairly short period of time, they break down and are no longer part of the atmosphere. These references, however, fail to teach solvents with optimized solubility parameters, together with desirable toxicity, and material compatibility. Specifically, these references do not suggest any advantages of using chlorine-containing ethers.
U.S. Pat. No. 5,273,592 discloses partially fluorinated ethers having a tertiary structure for solvent cleaning. The benefits of combining partially fluorinated ethers with hydrofluorochloro-ethers (HFCE's) or hydrobromochlorofluoro-alkenes (HBCFA's) for solvent applications are not suggested.
U.S. Pat. No. 4,999,127 teaches an azeotropic mixture of CHF2—CClF—O—CHF2, trans-1,2-dichloroethylene, and methanol. Some components of this mixture are toxic and flammable, and hence, not desirable as a safe second generation solvent replacement.
In short, the prior art has taught replacements to CFC's which only partially meet the requirements of a second generation solvent. There is thus a need for second generation replacement solvents that possess all required performance parameters.